The hemodynamic studies of the pulmonary microcirculation and its role in the regulation of the pulmonary blood flow have been limited by the techniques available and by the difficulty of making microvascular measurements. We have demonstrated that the density fluctuation is the result of the Fahraeus effect in pulmonary capillaries that are deformed by changes in ventilation or perfusion. Based on these studies, we can hypothesize that a zero density fluctuation can occur if the cyclic component of the pulmonary capillary pressure Pc equals that of the airway pressure PA under the condition of a constant transpulmonary pressure. Accordingly, we can use the density fluctuation and the measured PA to assess the cyclic Pc and subsequently through a mechanical simulation the hemodynamic properties of the pulmonary arterial, capillary, and venous vasculatures. Our long term objective is to use this methodology to study how mechanical, physiological, and pathological changes of the lung affect the regulation of the pulmonary blood flow. The specific aim of the first project is on the development of this density methodology using isolated, perfused lungs of the rabbits. Then we will study the effect of transpulmonary and vascular pressure and pulmonary edema on the hemodynamics of the pulmonary circulation. In the second project, we shall measure the density variation following an occlusion of the pulmonary artery or a partial occlusion of the pulmonary veins. This measurement provides a verification of the simulation and an assessment on the change in the pulmonary capillaries following the occlusion. We shall study in the third project the density variation induced by the Mueller maneuver aiming to develop an in vivo method to assess the effect of ventilation on the right ventricle, the lung, and the left atrium.